1. Technical Field
The present invention relates to a communication method, a communication apparatus, and a communication system for communication among a plurality of communication apparatuses using a common transmission channel by sharing a communication band.
2. Background Art
In communication systems using power line communications (PLC) or wireless LAN (Local Area Network), a plurality of communication apparatuses communicate with one another using a common transmission channel by sharing a communication band. Accordingly, when the plurality of communication apparatuses simultaneously transmits signals to the transmission channel, interference occurs and thus it is difficult to perform communication. Therefore, it is necessary to avoid such collision of access from the plurality of communication apparatuses to the transmission channel.
As one of methods for avoiding such collision of access, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) has been known. In the CSMA/CA, each communication apparatus recognizes the usage of a transmission channel, and transmits a signal during a period in which other communication apparatuses are not using the transmission channel. Specifically, the communication apparatus detects whether or not there is a carrier (carrier signal) of the signal transmitted by the other communication apparatus. When the communication apparatus detects the carrier, the communication apparatus waits until the communication performed by the other communication apparatus is completed. Meanwhile, if the communication apparatus does not detect the carrier, the communication apparatus tries to transmit a signal after additionally waiting for a waiting time called a back-off time. By determining the back-off time on the basis of a random value or the like, even when a plurality of communication apparatuses share the same transmission channel, it is possible to reduce the occurrence frequency of collisions and further to raise the use efficiency of the transmission channel.
As a known technique related to the communication using the CSMA/CA, it has been known to perform multiplexing of frames to improve transmission efficiency of the frames (e.g., see JP-A-2006-246035). In addition, a technique for allowing CSMA and TDMA (Time Division Multiple Access) to coexist on the same transmission channel has been known (e.g., see JP-A-2007-166104).
A communication network connected to a power line communication apparatus performing the known power line communication includes one or more power line communication apparatuses as slaves, and power line communication apparatuses as masters for managing them. There is an upper limit in the total number of the power line communication apparatuses as masters and the power line communication apparatuses as slaves which are connectable to one communication network or a common transmission channel, for example, it is limited to 255. In addition, it is important to allow relatively low speed power line communication apparatuses, which do not need high speed control data and commands, to coexist with known high speed power line communication apparatuses without having an influence on the performance of the high speed power line communication apparatuses.
However, according to used applications, there is a case where a desired process cannot be performed in the range of the limited number of the power line communication apparatuses in such one communication network.
For example, the known power line communication apparatus as master or slaves needs to detect beacon signals periodically transmitted from the power line communication apparatuses as the masters, or each slave needs to correctly recognize a time slot assigned to itself. Accordingly, a microcomputer with high process throughput or a high-performance logic circuit is necessary to perform communication control. Such a microcomputer with high process performance or the like is expensive.
If it is necessary to additionally install more power line communication apparatuses, it is preferable that the power line communication apparatus can be additionally installed as inexpensively as possible. In this case, signal collision with signals transmitted by the other power line communication apparatuses has to be necessarily avoided in the same manner as the known power line communication apparatuses as master or slaves.